The purposes of this study are to evaluate the safety, antiviral activity, and clinical efficacy of ABT-538 at a dose of 600 mg. BID compared to placebo in patients who have had at least nine months of previous exposure to existing available antiretroviral agents. The double blind phase of the trial is completed when 191 patients have experienced disease progression. An optional open-label ABT-538 treatment extension is available to patients under certain conditions.